Power supply circuit and image forming apparatus

ABSTRACT

A power supply circuit includes a thermoelectric conversion element configured to generate electric power when it is differentially heated, an adjustable current circuit configured to draw a current from the thermoelectric conversion element and resultantly output a constant current over a period of time, a voltage conversion circuit configured to output a voltage based on the current output by the adjustable current circuit, and a control circuit configured to control the adjustable current circuit to change a target value of the constant current output by the adjustable current circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2017-120608, filed Jun. 20, 2017, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a power supply circuitand an image forming apparatus.

BACKGROUND

In the related art, there is a technology in which heat is generatedfrom a heating element such as a heater using electric power from an ACpower supply, and processing of a component, melting of a wax, and thelike are performed using the generated heat of the heating element. Forexample, in an image forming apparatus that performs printing inaccordance with a print request, a dye material (toner) is melted so asto be fixed on a print medium by a fixing roller which is heated by ahigh temperature heating element. In this manner, the image formingapparatus forms an image on the print medium.

High electric power of about thousands of watts is required for heatingthe heating element to a high temperature to melt the toner. Inaddition, the heat of the heating element after the processing is endedis generally discharged into the air. A charging control device whichincludes a thermoelectric conversion element and a storage battery canbe provided. The thermoelectric conversion element generates electricpower when heated. A storage battery stores the electric power generatedby the thermoelectric conversion element.

In such a charging control device, driving a DC/DC converter is requiredfor drawing electric power from the thermoelectric conversion element.The current which can be drawn from the thermoelectric conversionelement is limited depending on the characteristics and a temperaturedifference across the thermoelectric conversion element. Thus, asituation in which electric power to be drawn from the thermoelectricconversion element is not increased, even though a control of increasingan output of the DC/DC converter is performed, may occur. In such acase, there is a problem in that efficiency of drawing electric powerfrom the thermoelectric conversion element is decreased.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram illustrating an example of a configuration of animage forming apparatus according to an embodiment.

FIG. 1B is a diagram illustrating an example of an overview of an entirecircuit configuration according to the embodiment.

FIG. 2 is a diagram illustrating an example of a configuration of apower supply circuit according to the embodiment.

FIG. 3 is a diagram illustrating an example of a configuration of aninsulating DC-DC circuit according to the embodiment.

FIG. 4 is a diagram illustrating an example of a configuration of aheater control circuit according to the embodiment.

FIG. 5 is a diagram illustrating an example of a configuration of anA-CC circuit according to the embodiment.

FIG. 6 is a diagram illustrating an example of a configuration of aDC-DC circuit according to the embodiment.

FIG. 7 is a diagram illustrating a relationship between a paperdischarge timing, a temperature, an A-CC driving pulse, and a generatedpower current.

FIG. 8 is a diagram illustrating an example of characteristics of athermoelectric conversion element.

FIGS. 9A to 9C are diagrams illustrating examples of a change of thegenerated power current when ON duty cycle of the A-CC driving pulse isswitched.

FIG. 10 is a diagram illustrating a relationship between the generatedpower current and the ON duty cycle.

FIG. 11 is a diagram illustrating the relationship between the generatedpower current and the ON duty cycle.

FIG. 12 is a diagram illustrating an example of an operation of acontrol circuit when electric power is taken out from the thermoelectricconversion element.

FIG. 13 is a diagram illustrating an example of an operation of thecontrol circuit when the electric power is supplied to a load circuit.

FIG. 14 is a diagram illustrating an operation of each of the circuitsin the power supply circuit.

DETAILED DESCRIPTION

Embodiments provide a power supply circuit having high efficiency and animage forming apparatus.

According to an embodiment, a power supply circuit includes athermoelectric conversion element configured to generate electric powerwhen it is differentially heated, an adjustable current circuitconfigured to draw a current from the thermoelectric conversion elementand resultantly output a constant current over a period of time, avoltage conversion circuit configured to output a voltage based on thecurrent output by the adjustable current circuit, and a control circuitconfigured to control the adjustable current circuit to change a targetvalue of the constant current output by the adjustable current circuit.

Hereinafter, an embodiment will be described with reference to thedrawings.

FIG. 1A is a diagram illustrating a configuration example of an imageforming apparatus 1 according to an embodiment.

The image forming apparatus 1 is, for example, a multifunctionperipheral (MFP) that performs various types of processing such as imageforming while transporting a recording medium such as a print mediumtherein. The image forming apparatus 1 charges a photoconductive drumand irradiates the photoconductive drum with light in accordance withimage data (print data) for printing, so as to form a latent image(electrostatic latent image) on the photoconductive drum. The imageforming apparatus 1 causes a toner (developer) to adhere to the latentimage formed on the photoconductive drum, and transfers the toner whichadheres to the latent image, onto a print medium, so as to forma tonerimage on the print medium. The image forming apparatus 1 nips the printmedium on which the toner image is formed, by one of a pair of fixingrollers 46 (which will be described later herein) and a thermal fixingbelt BT which is heated to a high temperature (which will be describedlater herein), so as to fix the adhered toner image onto the printmedium.

The image forming apparatus 1 acquires an image present on a medium in amanner that imaging is performed in an image sensor by using thereflected light of light with which the medium was irradiated, andcharges accumulated in the image sensor are read and the read chargesare converted into a digital signal representative of the image.

The image forming apparatus 1 includes a housing 11, a document stand12, a scanner unit 13, an automatic document feeder (ADF) 14, a paperfeeding cassette 15, a paper discharge tray 16, an image forming unit17, a transporting unit 18, a thermal fixing unit F, and athermoelectric conversion element 74.

The housing 11 is the main body for holding the document stand 12, thescanner unit 13, the ADF 14, the paper feeding cassette 15, the paperdischarge tray 16, the image forming unit 17, the transporting unit 18,the thermal fixing unit F, and the thermoelectric conversion element 74.

The document stand 12 is a part on which a medium O as an originaldocument to be scanned is placed. The document stand 12 includes a glassplate 31 and a space 33 therein. The medium O as the original documentis placed on the glass plate 31. The space 33 is positioned on a surfaceon an opposite side of a placement surface 32 of the glass plate 31, onwhich the medium O as the original document is placed.

The scanner unit 13 acquires an image from the medium P in accordancewith a control signal of a main controller 19 (which will be describedlater herein). The scanner unit 13 is disposed in the space 33 on thedocument stand 12 of the placement surface 32, which is an opposite sideof the placement surface 32. The scanner unit 13 includes an imagesensor, an optical element, an illumination equipment, and the like.

The image sensor is an imaging element in which pixels, in which lightis converted into an electric signal (image signal), are arranged alonga generally straight line. The image sensor is configured, for example,by a charge coupled device (CCD), a complementary metal oxidesemiconductor (CMOS), or other imaging elements.

The optical element forms an image in the pixels of the image sensorusing light received within a predetermined reading range. The readingrange of the optical element is a line-like region on the placementsurface 32 of the document stand 12. The optical element forms an imagein the pixels of the image sensor with light which is reflected by themedium P placed on the placement surface 32 of the document stand 12 andtransmitted through the glass plate 31.

Illumination equipment irradiates the medium O with light. Theillumination equipment includes a light source and a light guide forirradiating the medium O with light from the light source. Theillumination equipment irradiates a region including the reading regionof the optical element, with light emitted from the light source. Theirradiation is performed by the light guide.

When the medium O is placed on the placement surface 32 of the documentstand 12, the scanner unit 13 is driven by a driving mechanism (notillustrated) in a sub-scanning direction which is perpendicular to anarrangement direction (main scanning direction) of the pixels in theimage sensor and is parallel to the placement surface 32. The scannerunit 13 continuously acquires an image for each line by the imagesensor, while being driven in the sub-scanning direction. Thus, thescanner unit 13 acquires image data (original document image data) ofthe entirety of the medium O placed on the placement surface 32 of thedocument stand 12.

The ADF 14 is a mechanism for transporting the medium O. The ADF 14 isprovided on the document stand 12 to be freely closed or opened. The ADF14 takes a medium O disposed on a tray, in accordance with the controlof the main controller 19 (which will be described later). The ADF 14transports the taken medium O to a scanning location on the glass plate31 of the document stand 12.

If the medium O is transported by the ADF 14, the scanner unit 13 isdriven to a position facing a position of the glass plate 31 at whichthe medium O is located by the ADF 14. The scanner unit 13 continuouslyacquires an image from the medium O transported by the ADF 14, for eachline by the image sensor, and thus acquires image data (originaldocument image data) of the entirety of the medium O transported by theADF 14.

The paper feeding cassette 15 is a cassette for accommodating a printmedium P to be printed upon. The paper feeding cassette 15 is configuredso as to allow for a supply of the print medium P to be located thereinfrom the outside of the housing 11. For example, the paper feedingcassette 15 is configured so as to be allowed to be withdrawn from thehousing 11.

The paper discharge tray 16 is a tray for supporting the print medium Pdischarged from the image forming apparatus 1.

The image forming unit 17 is a printer that forms an image on a printmedium P under the control of the main controller 19 (which will bedescribed later herein). For example, the image forming unit 17 chargesthe drum, and forms a latent image depending on image data (print data)for printing, on the charged drum. The image forming unit 17 causes atoner to adhere to the latent image formed on the drum and transfers thetoner adhering to the latent image, onto the print medium P. Thus, theimage forming unit 17 forms an image on the print medium P. The imageforming unit 17 includes, for example, a drum 41, an exposure machine42, a developing machine 43, a transfer belt 44, a pair of transferrollers 45, and the thermal fixing unit F, as illustrated in FIG. 1A.

The drum 41 is a photoconductive drum which is formed to have acylindrical shape. The drum 41 is provided to come into contact with thetransfer belt 44. The surface of the drum 41 is uniformly charged by acharging charger (not illustrated). The drum 41 rotates at a constantspeed by a driving mechanism (not illustrated).

The exposure machine 42 forms an electrostatic latent image on thecharged drum 41. The exposure machine 42 irradiates the surface of thedrum 41 with a laser beam by a light emitting element or the like inaccordance with the print data, and thus forms an electrostatic latentimage on the surface of the drum 41. The exposure machine 42 includes alight emitting unit and an optical element.

The light emitting unit has a configuration in which light emittingelements for emitting light in accordance with an electric signal (imagesignal) are arranged in line. Each of the light emitting elements in thelight emitting unit emits light having a wavelength which allows alatent image to be formed on the charged drum 41. The light emitted fromthe light emitting unit forms an image on the surface of the drum 41 bythe optical element.

The developing machine 43 causes the toner (developer) to adhere to theelectrostatic latent image formed on the drum 41. Thus, the developingmachine 43 forms a toner image on the surface of the drum 41.

The drum 41, the exposure machine 42, and the developing machine 43 ofthe image forming unit 17 are provided for each of colors of cyan,magenta, yellow, and black, for example. In this case, each of aplurality of developing machines 43 holds a different color toner.

The transfer belt 44 is a member that receives a toner image formed onthe surface of the drum 41 and causes the toner image to be transferredto a print medium P. The transfer belt 44 is moved by the rotation ofthe roller. The transfer belt 44 receives the toner image formed on thedrum 41 at a position in contact with the drum 41, and moves thereceived toner image to the pair of transfer rollers 45.

The pair of transfer rollers 45 is configured to interpose the transferbelt 44 and a print medium P between them. The pair of transfer rollers45 cause the toner image on the transfer belt 44 to be transferred tothe print medium P.

The thermal fixing unit F includes the pair of fixing rollers 46, athermal fixing belt BT which is formed in an endless shape, a firstinductor L1, a heater control circuit 73. In this example, one of thepair of fixing rollers 46 and the first inductor L1 are arranged withinthe perimeter of the thermal fixing belt BT as shown in FIG. 1A. Theouter peripheral surface of the other of the pair of fixing rollers 46comes into contact with the outer peripheral surface of the thermalfixing belt BT and presses the thermal fixing belt BT toward the one ofthe pair of fixing rollers 46. The print medium P on which a toner imageis transferred passes between the outer peripheral surface of thethermal fixing belt BT and the outer peripheral surface of the other ofthe pair of the fixing rollers 46. The thermal fixing belt BT is heatedby induction heating using the first inductor L1 under control of theheater control circuit 73. The outer peripheral surface of the thermalfixing belt BT and the outer peripheral surface of the other of the pairof the fixing rollers 46 press on the print medium P nipped therebetweenin the heated state, and thus fixes the transferred toner image onto theprint medium P. That is, the thermal fixing unit F fixes the tonerimage, and thus causes a fixed image to be formed on the print medium Punder control of the heater control circuit 73.

The transporting unit 18 transports a print medium P. The transportingunit 18 includes a transporting path and a sensor. The transporting pathis configured by a plurality of guides and a plurality of rollers. Thesensor detects a transportation position of a print medium P on thetransporting path. The transporting path is a path through which a printmedium P is transported. Transporting rollers, which are arranged alongthe transporting path are rotated by a motor which operates under thecontrol of the main controller 19 (which will be described later). Thus,the print medium P is transported along the transporting path. Some ofthe plurality of guides are moved by a motor which operates under thecontrol of the main controller 19 (which will be described later), andthus causes the transporting path for transporting a print medium P tobe switched.

For example, as illustrated in FIG. 1A, the transporting unit 18includes a feeding roller 51, a fed paper transporting path 52, a paperdischarging path 53, and a reversal transporting path 54.

The feeding roller 51 picks up an upper most print medium P among printmedia stacked in the paper feeding cassette 15 and feeds it to the fedpaper transporting path 52.

The fed paper transporting path 52 is a transporting path fortransporting the print medium P which is taken out from the paperfeeding cassette 15 by the feeding roller 51 to the image forming unit17.

The paper discharging path 53 is a transporting path for discharging aprint medium P on which an image is formed by the image forming unit 17,from the housing 11. The print medium P discharged on the paperdischarging path 53 is discharged to the paper discharge tray 16.

The reversal transporting path 54 is a transporting path for supplying aprint medium P to the image forming unit 17 again in a state where, forexample, the front and the back, or the front and the rear of the printmedium P on which an image is formed by the image forming unit 17 arereversed.

FIG. 1B is a schematic diagram of an entire circuit configuration of theimage forming apparatus 1 according to the embodiment. As shown in FIG.1B, the image forming apparatus 1 includes, for example, a first powersupply circuit 81, a second power supply circuit 82, a third powersupply circuit 83, a stand-by power supply circuit 20, the heatercontrol circuit 73, the main controller 19, and the thermoelectricconversion element 74.

The thermoelectric conversion element 74 is provided at a position on adownstream side and in the vicinity of the thermal fixing unit F in thepaper transporting direction of the paper discharging path 53. Thethermoelectric conversion element generates electric power upon beingheated. The thermoelectric conversion element 74 is, for example,thermocouple in which two different kinds of metal or semiconductors arebonded to each other. The thermoelectric conversion element 74 is heatedby heat of a thermal fixing load 47 such as a paper and toner, passingthrough the thermoelectric conversion element 74, after being heated bythe thermal fixing unit F and heat transmitted through the air from thethermal fixing unit F, and resultantly generates electric power. Thatis, the thermoelectric conversion element 74 generates electric powerfrom the heat.

The first power supply circuit 81 is configured to receive a powersupply from an AC power supply E and supply power by 1200 W to theheater control unit 73, for example.

The second power supply circuit 82 is configured to receive a powersupply from the AC power supply E and supply power by 300 W to thetransport unit 18, for example.

The third power supply circuit 83 is configured to receive a powersupply from the AC power supply E and supply power by 100 W to the maincontroller 19, for example.

In the main controller 19, a stand-by module SB is provided and controlspower supply to the main controller 19 during the image formingapparatus is in a sleep mode in which the supply of the electric powerto the heater control circuit 73 and a transport unit 18 is suspendedfor energy saving.

The stand-by power supply circuit 20 is configured to receive a powersupply from either one of the AC power supply E and the thermoelectricconversion element 74 and supply power by 1 W to the stand-by module SBin the main controller 19, for example.

The heater control circuit 73 is configured to control the inductionheating by the thermal fixing unit F in collaboration with the maincontroller 19.

In addition, the main controller 19 entirely controls the image formingapparatus 1. For example, the main controller 19 entirely performscontrol of the scanner unit 13, the ADF 14, the image forming unit 17,the transporting unit 18, and an operation input unit (not shown) of theimage forming apparatus 1.

The main controller 19 includes, for example, a CPU, a ROM, a RAM, and anonvolatile memory.

The CPU is a computing element (for example, a processor) that performscomputing processing. The CPU performs various types of processing basedon data such as a program, which is stored in the ROM. The CPU functionsas a control unit which can perform various operations, by executing aprogram stored in the ROM. The CPU inputs print data for forming animage on a print medium P to the image forming unit 17. The CPU inputs atransporting control signal for an instruction to transport a printmedium P, to the transporting unit 18.

The ROM is a nonvolatile read only memory. The ROM stores a program anddata used in the program, for example.

The RAM is a volatile memory functioning as a working memory which canbe read from and written to. The RAM temporarily stores data during theprocessing of the CPU. The RAM temporarily stores a program executed bythe CPU.

The nonvolatile memory is a storage medium (storage unit) which iscapable of storing various types of information. The nonvolatile memorystores a program and data used in the program, for example. As thenonvolatile memory, for example, a solid state drive (SSD), a hard diskdrive (HDD), or another storage device is provided. Instead of thenonvolatile memory, a memory IF such as a card slot into which a storagemedium such as a memory card can be inserted may be provided.

The stand-by power supply circuit 20 is configured to supply electricpower to the stand-by module SB. FIG. 2 is a circuit diagramillustrating a configuration of the stand-by power supply circuit 20.The stand-by power supply circuit 20 receives AC power supplied from theexternal AC power supply E. The stand-by power supply circuit 20converts the supplied AC power into electric power having a voltage inaccordance with a stand-by module SB, and supplies the electric powerobtained by the conversion to the stand-by module SB.

The stand-by power supply circuit 20 includes a full-wave rectifyingcircuit 71, an insulating DC-DC circuit 72, an adjustable constantcurrent (A-CC) circuit 75, a secondary battery 76, a residual amountdetection circuit 77, a DC-DC circuit 78, and a control circuit 79.

The full-wave rectifying circuit 71 is a circuit configured to performfull-wave rectification of the AC power input from the AC power supply Eand to supply a ripple voltage to a circuit at a subsequent stage of thestand-by power supply circuit 20. For example, the full-wave rectifyingcircuit 71 is configured by a plurality of diodes and includes arectifying bridge configured to receive the input AC power.

The insulating DC-DC circuit 72 is a converter that supplies DC electricpower to the stand-by module SB using the ripple voltage from thefull-wave rectifying circuit 71.

FIG. 3 is a diagram illustrating a configuration example of theinsulating DC-DC circuit 72. The insulating DC-DC circuit 72 is, forexample, a flyback converter. The insulating DC-DC circuit 72 supplieselectric power to the secondary side thereof which is insulated from theprimary side thereof, to which electric power is supplied from the powersource. The insulating DC-DC circuit 72 includes a first capacitor C1, aprimary winding T1, a secondary winding T2, a first switching elementSW1, a rectifying diode D, a second capacitor C2, a first pulse widthmodulation (PWM) pulse generator 81, a first voltage detection circuitV1, and a photocoupler 82.

The first capacitor C1 smooths the input ripple voltage. The primarywinding T1 functions as a primary transformer winding. The secondarywinding T2 is electromagnetically coupled with the primary winding T1and functions as a secondary transformer winding. The first switchingelement SW1 performs switching between a conduction state (ON or closed)and a non-conduction state (OFF or open) in accordance with a pulsesignal input from the first PWM pulse generator 81, and thus causes thecurrent flowing in the primary winding T1 to switch between ON and OFF.The rectifying diode D rectifies the current generated in the secondarywinding T2. The second capacitor C2 smooths the voltage generated in thesecondary winding T2.

The first PWM pulse generator 81 inputs a pulse signal to the firstswitching element SW1, under the control of the control circuit 79.Thus, the first PWM pulse generator 81 performs switching of the firstswitching element SW1 between ON and OFF states. The first voltagedetection circuit V1 detects the voltage of the second capacitor C2. Thephotocoupler 82 applies feedback to the first PWM pulse generator 81 inaccordance with a detection result of the first voltage detectioncircuit V1.

In the above configuration, if the first switching element SW1 turns ON,a current flows in the primary winding T1 and electric energy isconverted into magnetic energy. At this time, a reverse voltage isapplied to the rectifying diode D on the secondary side of the circuit,and thus electric power is not delivered to the secondary side of thecircuit. Then, if the first switching element SW1 turns OFF, a currentflows into the second capacitor C2 via the rectifying diode D byaccumulated magnetic energy, and electric power is stored in the secondcapacitor C2. Electric power stored in the second capacitor C2 issupplied to the stand-by module SB at the subsequent stage of thestand-by power supply circuit 20, as DC electric power.

The electric power delivered to the secondary side is determined basedon the current flowing in the primary winding T1. That is, the electricpower delivered to the secondary side is determined by the time periodwhen the first switching element SW1 is in an ON state. The first PWMpulse generator 81 increases the current flowing in the primary windingT1 and increases the electric power delivered to the secondary side bywidening the pulse width, i.e., the duration of the first pulse, tothereby increase the “ON” duty cycle. The first voltage detectioncircuit V1 and the photocoupler 82 perform feedback of an output voltageto the first PWM pulse generator 81. The first PWM pulse generator 81controls the pulse width of the pulse signal for driving the firstswitching element SW1, based on feedback from the first voltagedetection circuit V1 and the photocoupler 82, and thus holds the outputvoltage constant.

The insulating DC-DC circuit 72 may also be a DC-DC converter having aninsulating function. For example, the insulating DC-DC circuit 72 mayalso be configured by an LLC circuit, an insulating forward circuit, andan insulating double forward circuit.

The heater control circuit 73 is connected to the AC power supply E. Theheater control circuit 73 is a circuit configured to generate anelectromagnetic wave using AC power from the AC power supply E and toinduce a current in the first inductor L1 and thereby heat the thermalfixing belt BT by induction heating.

FIG. 4 is a diagram illustrating a configuration example of the heatercontrol circuit 73. The heater control circuit 73 is, for example, ahalf bridge circuit. The heater control circuit 73 includes a full-waverectifying circuit 83, a third capacitor C3, a fourth capacitor C4, asecond switching element SW2, a third switching element SW3, the firstinductor L1, and a half bridge alternating pulse generator 84.

The full-wave rectifying circuit 83 is a circuit configured to performfull-wave rectification of the AC power input from the AC power supply Eand to supply a ripple voltage to a circuit at the subsequent stage. Forexample, the full-wave rectifying circuit 83 is configured by aplurality of diodes and includes a rectifying bridge configured toreceive the input AC power.

The third capacitor C3 and the fourth capacitor C4 are connected inseries to a DC terminal of the full-wave rectifying circuit 83. Thesecond switching element SW2 and the third switching element SW3 areconnected to the DC terminal of the full-wave rectifying circuit 83 soas to be in parallel with the series connection of the third capacitorC3 and the fourth capacitor C4. The first inductor L1 is connectedbetween a connection point of the second switching element SW2 and thethird switching element SW3 and a connection point of the thirdcapacitor C3 and the fourth capacitor C4.

The half bridge alternating pulse generator 84 inputs a pulse signal tothe second switching element SW2 and inputs a pulse signal having alogical value reversed to that of the pulse signal input to the secondswitching element SW2, to the third switching element SW3 under thecontrol of the control circuit 79. Thus, the half bridge alternatingpulse generator 84 performs switching of the second switching elementSW2 and the third switching element SW3 between the conduction state(ON) and the non-conduction state (OFF).

In the above configuration, the second switching element SW2 and thethird switching element SW3 alternately turn ON and OFF. Thus, electricpower of a DC voltage supplied from the full-wave rectifying circuit 83is converted into high-frequency AC power. The heater control circuit 73performs induction heating using the high-frequency AC power. Ininduction heating, an eddy-current is generated in a conductor of thethermal fixing belt BT, and heat is generated by virtue of theresistance of the conductor in the thermal fixing belt BT. Thus, theheater control circuit 73 causes the first inductor L1 to generate theelectromagnetic field and heats the thermal fixing belt BT using thegenerated electromagnetic field.

The half bridge alternating pulse generator 84 generates a square wave(pulse signal) corresponding to 50% of the pulses of a predeterminedfrequency, by using a timer, a CR time constant, and the like. The halfbridge alternating pulse generator 84 inputs the generated pulse signalto a gate of one switching element and inputs a pulse signal obtained byreversing a logical value, to a gate of the other switching element. Thehalf bridge alternating pulse generator 84 provides a dead time when apulse signal for OFF is input to the two switching elements, so as tocause the two switching elements not to simultaneously turn ON.

The half bridge alternating pulse generator 84 changes a frequencygenerated by the timer, under the control of the control circuit 79. Ifthe frequency (transmission frequency) generated by the timer isincreased, a high-frequency current flowing in the first inductor L1 isdecreased and the quantity of heat generated in the thermal fixing beltBT is decreased. If the frequency (transmission frequency) generated bythe timer is decreased, the high-frequency current flowing in the firstinductor L1 is increased and the quantity of heat generated in thethermal fixing belt BT is increased. That is, the half bridgealternating pulse generator 84 controls heat generation of the thermalfixing belt BT under the control of the control circuit 79.

The heater control circuit 73 is configured to be capable of heating thethermal fixing belt BT. For example, the heater control circuit 73 mayalso be configured by a full bridge inverter, a semi-E class inverter,and a push-pull inverter.

The A-CC circuit 75 draws electric power generated in the thermoelectricconversion element 74. The A-CC circuit 75 is a variable constantcurrent circuit configured to draw a constant current having a value inaccordance with the control of the control circuit 79, from thethermoelectric conversion element 74.

FIG. 5 is a diagram illustrating a configuration example of the A-CCcircuit 75. The A-CC circuit 75 includes a second inductor L2, a fourthswitching element SW4, a fifth switching element SW5, a fifth capacitorC5, a second PWM pulse generator 85, a first synchronous rectificationpulse generator 86, and a current detection circuit A.

The fourth switching element SW4 and the fifth switching element SW5 areconnected in series between a pair of output terminals. The secondinductor is connected between a connection point of the fourth switchingelement SW4 and the fifth switching element SW5, and one of a pair ofinput terminals. The fifth capacitor C5 is connected in parallel with aseries connection of the fourth switching element SW4 and the fifthswitching element SW5, when viewed from the pair of output terminals.

The second PWM pulse generator 85 inputs a pulse signal to the fourthswitching element SW4 under the control of the control circuit 79. Thus,the second PWM pulse generator 85 performs switching of the fourthswitching element SW4 between the conduction state (ON) and thenon-conduction state (OFF).

The first synchronous rectification pulse generator 86 inputs a pulsesignal to the fifth switching element SW5 in accordance with a signalsupplied from the second PWM pulse generator 85. Thus, the firstsynchronous rectification pulse generator 86 switches the fifthswitching element SW5 between the conduction state (ON) and theconduction state (OFF) through a body diode. The fifth switching elementSW5 normally causes a current to intermittently flow in a reversedirection from its source toward its drain. Thus, the fifth switchingelement SW5 is in a state where a current flows via the body diode, notin the OFF state. A circuit operation is established by causing acurrent to flow in the body diode, but a 0.7 V loss occurs which is theforward voltage of the body diode. Therefore, if the fifth switchingelement SW5 is caused to be in the ON state so as to allow conductionwithout current passing through the body diode, it is possible to reduceloss in the fifth switching element SW5. This is generally referred toas synchronous rectification control.

The second PWM pulse generator 85 supplies a signal to the firstsynchronous rectification pulse generator 86 such that the firstsynchronous rectification pulse generator 86 inputs a pulse signalhaving a logical value reversed to that of the pulse signal input to thefourth switching element SW4, to the fifth switching element SW5. Thus,the second PWM pulse generator 85 causes the fourth switching elementSW4 and the fifth switching element SW5 to alternately turn ON and OFF,i.e., allow current to pass therethrough in the ON state, and preventcurrent passing therethrough during the OFF state. The second PWM pulsegenerator 85 provides a dead time when all of the fourth switchingelement SW4 and the fifth switching element SW5 are in the OFF state, inthe pulse signal so as to cause the fourth switching element SW4 and thefifth switching element SW5 not to simultaneously turn ON.

The current detection circuit A is connected in series with the secondinductor L2 and the fourth switching element SW4, at a location betweenthe pair of the input terminals. The current detection circuit A detectsa current value of a current generated by electric power which isgenerated by the thermoelectric conversion element 74.

In the above-described configuration, if the fourth switching elementSW4 is in the ON state, a current flows along the path of one inputterminal, the second inductor L2, the fourth switching element SW4, thecurrent detection circuit A, and the other input terminal. The currentin this path causes magnetic energy to be stored in the second inductor.

Then, if the fourth switching element SW4 turns OFF and the fifthswitching element turns ON in a state where magnetic energy is stored inthe second inductor, a current flows along the path of the secondinductor L2, the fifth capacitor C5, and the current detection circuitA. The current in this path causes magnetic energy in the secondinductor L2 to be converted into charge energy in the fifth capacitorC5. The charge energy stored in the fifth capacitor C5 is charged in thesecondary battery 76.

When the fourth switching element SW4 is in the ON state, the currentdetection circuit A detects a value of a current (generated powercurrent) flowing through, in the order of, the one input terminal, thesecond inductor L2, the fourth switching element SW4, and the otherinput terminal. The current detection circuit A supplies the detectionresult to the control circuit 79. When the fourth switching element SW4is in the OFF state, the current detection circuit A detects a value ofa current (generated power current) flowing through in the order of theone input terminal, the second inductor L2, the fifth switching elementSW5, the fifth capacitor C5, and the other input terminal. The currentdetection circuit A supplies the detection result to the control circuit79. The control circuit 79 supplies a control signal to the second PWMpulse generator 85 in accordance with the detection result from thecurrent detection circuit A.

The second PWM pulse generator 85 adjusts the pulse width of a pulsesignal input to the fourth switching element SW4, based on the controlsignal supplied from the control circuit 79. Thus, it is possible toperform constant current control to cause a constant current valueselected by the control circuit 79 to flow. That is, the second PWMpulse generator 85 adjusts the pulse width of a pulse signal input tothe fourth switching element SW4 such that a constant current having atarget value in accordance with the control signal supplied from thecontrol circuit 79 flows.

The secondary battery 76 is a storage battery that stores electric powersupplied from the A-CC circuit 75 and supplies the stored electric powerto other circuits. For example, the secondary battery 76 supplieselectric power to the DC-DC circuit 78 at a subsequent stage of thecontrol circuit 20.

The residual amount detection circuit 77 detects the residual amount ofthe electric power stored in the secondary battery 76 and supplies thedetection result to the control circuit 79.

The DC-DC circuit 78 is a converter that converts a voltage of electricpower supplied from the secondary battery 76 into a voltage required bythe stand-by module SB and supplies the DC electric power to thestand-by module SB.

FIG. 6 is a diagram illustrating a configuration example of the DC-DCcircuit 78. The DC-DC circuit 78 includes a third inductor L3, a sixthswitching element SW6, a seventh switching element SW7, a sixthcapacitor C6, a third PWM pulse generator 87, a second synchronousrectification pulse generator 88, and a second voltage detection circuitV2.

The sixth switching element SW6 and the seventh switching element SW7are connected in series between the pair of output terminals. The thirdinductor is connected at a location between a connection point of thesixth switching element SW6 and the seventh switching element SW7 andone of the pair of the input terminals. The sixth capacitor C6 isconnected in parallel with a series connection of the sixth switchingelement SW6 and the seventh switching element SW7, when viewed from thepair of output terminals.

The third PWM pulse generator 87 inputs a pulse signal to the sixthswitching element SW6 under the control of the control circuit 79. Thus,the third PWM pulse generator 87 switches the sixth switching elementSW6 between the conduction state (ON) and the non-conduction state(OFF).

The second synchronous rectification pulse generator 88 inputs a pulsesignal to the sixth switching element SW6 in accordance with a signalsupplied from the third PWM pulse generator 87. Thus, the secondsynchronous rectification pulse generator 88 switches the seventhswitching element SW7 between the conduction state (ON) and theconduction state (OFF) through a body diode.

The third PWM pulse generator 87 supplies a signal to the secondsynchronous rectification pulse generator 88 such that the secondsynchronous rectification pulse generator 88 inputs a pulse signalhaving a logical value reversed to that of the pulse signal input to thesixth switching element SW6, to the seventh switching element SW7. Thus,the third PWM pulse generator 87 causes the sixth switching element SW6and the seventh switching element SW7 to alternately turn ON and OFF.The third PWM pulse generator 87 provides a dead time when all of thesixth switching element SW6 and the seventh switching element SW7 are inthe OFF state in the pulse signal so as to cause the sixth switchingelement SW6 and the seventh switching element SW7 not to simultaneouslyturn ON.

The second voltage detection circuit V2 is connected in parallel withthe sixth capacitor at a portion between the pair of output terminals.

In the above-described configuration, if the sixth switching element SW6is in the ON state, a current flows in the path of one input terminal,the third inductor L3, the sixth switching element SW6, and the otherinput terminal. The current in this path causes magnetic energy to bestored in the third inductor.

Then, if the sixth switching element SW6 turns OFF and the seventhswitching element turns ON in a state where magnetic energy is stored inthe third inductor, a current flows in the path of the third inductor L3and the sixth capacitor C6. The current in this path causes magneticenergy in the third inductor L3 to be converted into charge energy inthe sixth capacitor C6. The charge energy stored in the sixth capacitorC6 is supplied to the stand-by module SB.

The second voltage detection circuit V2 detects a voltage of the sixthcapacitor, and supplies the detection result to the third PWM pulsegenerator 87. The third PWM pulse generator 87 performs a control basedon the detection result of the voltage in the second voltage detectioncircuit V2, such that the voltage in the sixth capacitor is equal to avoltage selected in accordance with the control of the control circuit79. Thus, the DC-DC circuit 78 supplies DC electric power at the voltageselected in accordance with the control of the control circuit 79 to thestand-by module SB using the electric power supplied from the secondarybattery 76.

The seventh switching element SW7 is, for example, an N-type MOSFET.When the N-type MOSFET is in the ON state, a current flows from thedrain thereof toward the source thereof. When the N-type MOSFET is inthe OFF state, the N-type MOSFET operates as a body diode in which acurrent flows from the source thereof toward the drain thereof. If theN-type MOSFET turns ON in this state, the N-type MOSFET operates as aswitch at a threshold voltage which is lower than a voltage when theN-type MOSFET operates as the diode.

For example, when a voltage applied to the body diode of the seventhswitching element SW7 is 1.5 V, conduction resistance when the seventhswitching element SW7 is in the ON state is 0.01Ω, and if a currentflowing at this time is 1 A, the resulting potential differencesatisfies V=1 A×0.01 Ω=0.01 V. That is, the voltage is lower than thevoltage generated in the body diode. As a result, it is possible todecrease conduction loss. As described above, a case where a MOSFET isconnected in a reverse direction and is used instead of a diode in orderto decrease the conduction loss is generally referred to as synchronousrectification.

The control circuit 79 controls an operation of the insulating DC-DCcircuit 72, an operation of the heater control circuit 73, an operationof the A-CC circuit 75, and an operation of the DC-DC circuit 78.Specifically, the control circuit 79 controls an output voltage of theinsulating DC-DC circuit 72 by inputting a control signal to the firstPWM pulse generator 81. The control circuit 79 controls the quantity ofheat to be generated in the thermal fixing belt BT by the heater controlcircuit 73, by inputting a control signal to the half bridge alternatingpulse generator 84 of the heater control circuit 73. The control circuit79 controls a current flowing in the second inductor L2 of the A-CCcircuit 75, by inputting a control signal to the second PWM pulsegenerator 85 of the A-CC circuit 75. The control circuit 79 controls anoutput voltage of the DC-DC circuit 78 by inputting a control signal tothe third PWM pulse generator 87 of the DC-DC circuit 78.

Next, a control of the operation of the A-CC circuit 75 by the controlcircuit 79 will be described in detail.

FIG. 7 is a diagram illustrating a relationship between a paperdischarge timing, a temperature, an A-CC driving pulse, and a generatedpower current. The horizontal axis indicates time. Vertical axesrespectively indicate the paper discharge timing, the temperature, theA-CC driving pulse, and the generated power current in that order fromthe top. Here, an example in which the electrophotographic image formingapparatus 1 discharges three sheets of print media after printingthereon is used.

The paper discharge timing indicates a timing when the print medium isdischarged from the thermal fixing unit F in the image forming apparatus1. A speed at which the print medium is discharged is, for example, onesheet per second. The temperature indicates a temperature detected by atemperature sensor (not illustrated) that detects the temperature of theouter peripheral surface of the thermal fixing belt BT or thetemperature of the outer peripheral surface of the other of the pair ofthe fixing rollers 46. The A-CC driving pulse is a signal used when thesecond PWM pulse generator 85 in the A-CC circuit 75 controls the fourthswitching element SW4. The generated power current indicates a value ofa current detected by the current detection circuit A in the A-CCcircuit 75.

For example, if the thermal capacity of the thermoelectric conversionelement 74 is very small, a temperature difference occurs between theopposed ends of the thermoelectric conversion element 74 caused by theair which is discharged along with the print medium. The temperaturedifference occurring between the opposed ends of the thermoelectricconversion element 74 is increased as the print medium is passing, andis slowly decreased after the print medium passes. The temperaturedifference occurring between the opposed ends of the thermoelectricconversion element 74 is increased whenever the number of dischargedprint media is increased.

The control circuit 79 controls the A-CC driving pulse to aim atobtaining a current which is substantially proportional to thetemperature difference occurring between or across the opposed ends ofthe thermoelectric conversion element 74, from the thermoelectricconversion element 74 as the generated power current. The controlcircuit 79 controls the timing for switching fourth switching elementSW4 of the A-CC circuit 75 between ON and OFF, by adjusting the pulsewidth of the A-CC driving pulse. That is, the control circuit 79 adjuststhe pulse width of the A-CC driving pulse, and thus the A-CC circuit 75controls the current value of the generated power current from thethermoelectric conversion element 74. The control circuit 79 receivesthe current value of the generated power current from the A-CC circuit,and adjusts the pulse width of the A-CC driving pulse in accordance withthe received current value. Thus, the control circuit 79 controls theA-CC circuit 75 to receive a current having a target or desired currentvalue, from the thermoelectric conversion element 74.

If the temperature difference occurring between the opposed ends of thethermoelectric conversion element 74 is small, the obtained generatedpower current is reduced. Thus, electric power required for driving theA-CC circuit 75 may be greater than electric power that can be obtainedfrom the thermoelectric conversion element 74. When it is not possibleto expect that electric power of a predetermined quantity or greater canbe generated by the thermoelectric conversion element 74, the controlcircuit 79 controls the A-CC circuit 75 to cause the A-CC driving pulseto indicate the OFF state. When the A-CC driving pulse indicates the OFFstate, the current which flows via the body diode of the fourthswitching element SW4 in the A-CC circuit 75 is also zero. As a result,the generated current is zero.

For example, when a temperature which is equal to or higher than apredetermined temperature is not detected by the temperature sensor, thecontrol circuit 79 controls the A-CC circuit 75 to suspend generation ofthe A-CC driving pulse. The control circuit 79 may have a configurationof suspending generation of the A-CC driving pulse when the controlcircuit 79 recognizes that the operation of the heater control circuit73 is suspended, in order to recognize the operation of the heatercontrol circuit 73. That is, if the control circuit 79 operates theheater control circuit 73, and thus discharging of a print medium isstarted, the control circuit 79 generates the A-CC driving pulse so asto operate the A-CC circuit 75. For example, the control circuit 79operates the A-CC circuit 75 with a pulse having a frequency of 100 kHzand a pulse period of about 10 μsec. That is, the control circuit 79supplies pulses of about the sixth power of ten to the A-CC circuit 75during a period when one print medium is discharged. Thus, it ispossible to control the A-CC circuit 75 at a very small resolution.

Next, characteristics of the thermoelectric conversion element 74 willbe described.

FIG. 8 is a diagram illustrating an example of the characteristics ofthe thermoelectric conversion element 74. The graph in FIG. 8 indicatesa value of a current which can be taken out, i.e., drawn from, thethermoelectric conversion element 74, with respect to the pulse width,that is, the ON duty cycle of the A-CC driving pulse. The horizontalaxis in the graph in FIG. 8 indicates the length of the ON duty cycle ofthe A-CC driving pulse. The vertical axis in the graph indicates thevalue of the current which can be taken out from the thermoelectricconversion element 74.

The second PWM pulse generator 85 in the A-CC circuit 75 generates theA-CC driving pulse of a desired amplitude for a selectable durationbased on the ON duty cycle designated by the control circuit 79. If theON duty cycle of the A-CC driving pulse is small, the duration of thepulse is of a first relatively short time period, and the current takenout from the thermoelectric conversion element 74 is small. If the ONduty cycle of the A-CC driving pulse is long, the duration of the pulseis of a second time period, which is longer than that of the relativelyshort time period, and the current taken out from the thermoelectricconversion element 74 is large. However, the current taken out from thethermoelectric conversion element 74 is determined based on the maximumvalue of a current which can be taken out in accordance with thetemperature difference between the opposed ends of the thermoelectricconversion element 74. Thus, if the current taken out from thethermoelectric conversion element 74 reaches the maximum value, thecurrent taken out from the thermoelectric conversion element 74 isincreased no more even if the ON duty cycle of the A-CC driving pulse isincreased.

In the example in FIG. 8, when the temperature difference between theopposed ends of the thermoelectric conversion element 74 is 80 degreesC., the maximum value of the current that can be taken out from thethermoelectric conversion element 74 is 0.1 A. When the temperaturedifference between the opposed ends of the thermoelectric conversionelement 74 is 100 degrees C., the maximum value of the current that canbe taken out from the thermoelectric conversion element 74 is 0.18 A.When the temperature difference between the opposed ends of thethermoelectric conversion element 74 is 120 degrees C., the maximumvalue of the current that can be taken out from the thermoelectricconversion element 74 is 0.26 A. That is, the maximum value of thecurrent that can be taken out from the thermoelectric conversion element74 is increased in proportion to the temperature difference between theopposed ends of the thermoelectric conversion element 74 when thisdifference is increased.

As described above, because the maximum value of the current, that can,or is to be, taken out depends on the temperature difference between theopposed ends of the thermoelectric conversion element 74 and thefrequency of the fluctuation of the temperature difference between theopposed ends of the thermoelectric conversion element 74 is high, thecurrent to be taken out may be decreased or an operation may be unstablyperformed even though a circuit (constant current circuit) configured totake a constant current out is connected to the thermoelectricconversion element 74. Thus, as illustrated in FIGS. 2 and 5, the A-CCcircuit 75 which can adjust the current value of the current taken outfrom the thermoelectric conversion element 74 is connected to thethermoelectric conversion element 74.

Next, a method of controlling the A-CC circuit 75 by the control circuit79 will be described.

The control circuit 79 controls the ON duty cycle of the A-CC drivingpulse based on a change of the current value (generated power current)detected by the current detection circuit A of the A-CC circuit 75 whenthe ON duty cycle of the A-CC driving pulse for driving the A-CC circuit75 is changed. More specifically, the control circuit 79 performsswitching at three stages in an order of the duration of the ON dutycycle of the A-CC driving pulse. The control circuit 79 determineswhether the ON duty cycle of the A-CC driving pulse should be increased,reduced, or unchanged, based on determination of whether the current ofthe generated power has increased, not changed, or decreased.

FIGS. 9A to 9C illustrate examples of a change of the generated powercurrent when the ON duty cycle of the A-CC driving pulse is switched.The example will be described on the assumption that a generated powercurrent when the ON duty cycle, i.e., the ON pulse duration, of the A-CCdriving pulse is the smallest is the generated power current I1, agenerated power current when the ON duty cycle, i.e., the ON pulseduration, of the A-CC driving pulse is the next smallest is thegenerated power current I2, and a generated power current when the ONduty cycle, i.e., the ON pulse duration, of the A-CC driving pulse isthe largest is the generated power current I3.

For example, as illustrated in FIG. 9A, when the generated power currentis slowly increased with an increase of the ON duty cycle of the A-CCdriving pulse, it is estimated that generated power current which can betaken out from the thermoelectric conversion element 74 yet remains.Thus, when I1<I2<I3 is satisfied (Case 1), the control circuit 79increases the ON duty cycle, i.e., the ON pulse duration of the A-CCdriving pulse.

For example, as illustrated in FIG. 9B, when an increase of thegenerated power current with the increase of the ON duty cycle of theA-CC driving pulse is stopped in the middle of the increase, it isestimated that generated power current which can be taken out from thethermoelectric conversion element 74 reaches the maximum value. Thus,when I1<I2=I3 is satisfied (Case 2), the control circuit 79 determinesthat the ON duty cycle of the A-CC driving pulse is adequate, andmaintains the ON duty cycle of the A-CC driving pulse.

For example, as illustrated in FIG. 9C, when the generated power currentis not changed with the increase of the ON duty cycle of the A-CCdriving pulse, it is estimated that generated power current that can betaken out from the thermoelectric conversion element 74 has reached themaximum value and the ON duty cycle is too long. Thus, when I1=I2=I3 issatisfied (Case 3), the control circuit 79 decreases the ON duty cycleof the A-CC driving pulse.

When the current ON duty cycle is set as D1, the control circuit 79controls the ON duty cycle of the A-CC driving pulse, for example, by aprogram as follows.

If (I1<I2 and I2<I3) {A=1;}

Else if(I1<I2 and I2==I3) {A=2:}Else if(I1==I2 and I2==I3) {A=3;}Else {A=999;} // error

Case A:

A=1 {D2=D2+d;}

A=2 {D2=D2;}

A=3 {D2=D2−d;}

End case;

FIGS. 10 and 11 are diagrams illustrating a relationship between thegenerated power current and the ON duty cycle when the A-CC circuit 75is controlled by the control method of the A-CC circuit 75 illustratedin FIGS. 9A to 9C. FIG. 10 illustrates an example in which thetemperature difference between the opposed ends of the thermoelectricconversion element 74 is slowly increased. FIG. 11 illustrates anexample in which the temperature difference between the opposed ends ofthe thermoelectric conversion element 74 is slowly decreased. Horizontalaxes in FIGS. 10 and 11 indicate time. Vertical axes in FIGS. 10 and 11respectively indicate an ideal current curve, the generated powercurrent, and the A-CC driving pulse. The ideal current curve showscharacteristics of the temperature difference-generated power current ofthe thermoelectric conversion element 74. When the horizontal axis isset to be in a (millisecond) ms range, it is not possible to illustrateON and OFF of the A-CC driving pulse having a frequency of 100 kHz and aperiod of 10 μS, in the drawings. Thus, in the examples in FIGS. 10 and11, a coarse pulse is illustrated as the A-CC driving pulse forconvenience.

The control circuit 79 acquires the current value of the generated powercurrent while changing the ON duty cycle of the A-CC driving pulse by±Δd. The control circuit 79 determines whether the ON duty cycle of theA-CC driving pulse has increased, been maintained, or decreased inaccordance with any change of the acquired current value, while changingthe ON duty cycle of the A-CC driving pulse. A mode in which the controlcircuit 79 acquires the current value of the generated power current isreferred to as “a current detection mode”. A mode in which determinationof whether the control circuit 79 has increased, maintained, ordecreased the ON duty cycle of the A-CC driving pulse is performed, andthe ON duty cycle is changed based on the determination result isreferred to as “a microcomputer processing determination-and-settingchange mode”. The control circuit 79 alternately performs “the currentdetection mode” and “the microcomputer processingdetermination-and-setting change mode”, and thus sequentially changesthe ON duty cycle.

For example, the control circuit 79 acquires the current value of thegenerated power current from the current detection circuit A of the A-CCcircuit 75 while changing the ON duty cycle in a range of D1−Δd toD1+Δd, during a period of a time t0 to a time t1. The control circuit 79determines whether to increase, maintain, or decrease the ON duty cycleduring a period of the time t1 to a time t2, based on the acquiredcurrent value. The control circuit 79 changes the ON duty cycle based onthe determination result, at the time t2. In the example in FIG. 10, thecontrol circuit 79 determines to increase the ON duty cycle during theperiod of the time t1 to the time t2. The control circuit 79 changes theON duty cycle from ON duty cycle D1 to ON duty cycle D2 which is greaterthan the ON duty cycle D1, at the time t2.

Then, the control circuit 79 acquires the current value of the generatedpower current from the current detection circuit A of the A-CC circuit75 while changing the ON duty cycle a range of D2−Δd to D2+Δd, during aperiod of the time t2 to a time t3. The control circuit 79 determineswhether to increase, maintain, or decrease the ON duty cycle during aperiod of time t3 to a time t4, based on the acquired current value. Thecontrol circuit 79 changes the ON duty cycle based on the determinationresult, at the time t4. In the example in FIG. 10, the control circuit79 determines to maintain the ON duty cycle during the period of thetime t3 to the time t4. In this case, the control circuit 79 maintainsthe ON duty cycle D2 even after the time t4.

Then, the control circuit 79 acquires the current value of the generatedpower current from the current detection circuit A of the A-CC circuit75 while changing the ON duty cycle in a range of D2−Δd to D2+Δd, duringa period of the time t4 to a time t5. The control circuit 79 determineswhether to increase, maintain, or decrease the ON duty cycle during aperiod of the time t5 to a time t6, based on the acquired current value.The control circuit 79 changes the ON duty based on the determinationresult, at the timing t6. In the example in FIG. 10, the control circuit79 determines to increase the ON duty cycle during the period of thetime t5 to the time t6. The control circuit 79 changes the ON duty cyclefrom ON duty cycle D2 to ON duty cycle D3 which is greater than the ONduty cycle D2, at the time t6.

Then, the control circuit 79 acquires the current value of the generatedpower current from the current detection circuit A of the A-CC circuit75 while changing the ON duty cycle in a range of D3−Δd to D3+Δd, duringa period of the time t6 to a time t7. The control circuit 79 determineswhether to increase, maintain, or decrease the ON duty cycle during aperiod of the time t7 to a time t8, based on the acquired current value.The control circuit 79 changes the ON duty cycle based on thedetermination result, at the time t8. In the example in FIG. 10, thecontrol circuit 79 determines to increase the ON duty cycle during theperiod of the time t7 to the timing t8. The control circuit 79 changesthe ON duty cycle from ON duty cycle D3 to ON duty cycle D4 which isgreater than the ON duty cycle D3, at the time t8.

Next, as illustrated in FIG. 11, an example in which the temperaturedifference between the opposed ends of the thermoelectric conversionelement 74 is slowly decreased will be described.

The control circuit 79 acquires the current value of the generated powercurrent from the current detection circuit A of the A-CC circuit 75while changing the ON duty cycle in a range of D4−Δd to D4+Δd, during aperiod of a time t9 to a time t10. The control circuit 79 determineswhether to increase, maintain, or decrease the ON duty cycle during aperiod of the time t10 to a time t11, based on the acquired currentvalue. The control circuit 79 changes the ON duty cycle based on thedetermination result, at the time t11. In the example in FIG. 11, thecontrol circuit 79 determines to maintain the ON duty cycle during aperiod of the time t9 to the time t11. In this case, the control circuit79 maintains the ON duty D4 cycle even after the time t11.

Then, the control circuit 79 acquires the current value of the generatedpower current from the current detection circuit A of the A-CC circuit75 while changing the ON duty cycle in a range of D4−Δd to D4+Δd, duringa period of the time t11 to a time t12. The control circuit 79determines whether to increase, maintain, or decrease the ON duty cycleduring a period of the time t12 to a time t13, based on the acquiredcurrent value. The control circuit 79 changes the ON duty cycle based onthe determination result, at the time t13. In the example in FIG. 11,the control circuit 79 determines to decrease the ON duty cycle duringthe period of the time t12 to the time t13. The control circuit 79changes the ON duty cycle from ON duty cycle D4 to ON duty cycle D5which is smaller than the ON duty cycle D4, at the time t13.

Then, the control circuit 79 acquires the current value of the generatedpower current from the current detection circuit A of the A-CC circuit75 while changing the ON duty cycle in a range of D5−Δd to D5+Δd, duringa period of the time t13 to a time t14. The control circuit 79determines whether to increase, maintain, or decrease the ON duty cycleduring a period of the time t14 to a time t15, based on the acquiredcurrent value. The control circuit 79 changes the ON duty cycle based onthe determination result, at the time t15. In the example in FIG. 11,the control circuit 79 determines to maintain the ON duty cycle during aperiod of the time t14 to the time t15. In this case, the controlcircuit 79 maintains the ON duty cycle D5 even after the time t15.

Then, the control circuit 79 acquires the current value of the generatedpower current from the current detection circuit A of the A-CC circuit75 while changing the ON duty cycle in a range of D5−Δd to D5+Δd, duringa period of the time t15 to a time t16. The control circuit 79determines whether to increase, maintain, or decrease the ON duty cycleduring a period of the time t16 to a time t17, based on the acquiredcurrent value. The control circuit 79 changes the ON duty cycle based onthe determination result, at the time t17. In the example in FIG. 11,the control circuit 79 determines to decrease the ON duty cycle duringthe period of the timing t16 to the timing t17. The control circuit 79changes the ON duty cycle from ON duty cycle D5 to ON duty cycle D6which is smaller than the ON duty cycle D5, at the time t17.

With the above processing, the control circuit 79 can control thecurrent value of the generated power current to generally follow theideal current curve.

Next, the control of the stand-by power supply circuit 20 by the controlcircuit 79 will be described. FIG. 12 is a diagram illustrating anexample of an operation of taking electric power from the thermoelectricconversion element 74.

When image is formed on a print medium, the control circuit 79 performsinitial settings of various registers (ACT11).

The control circuit 79 inputs a control signal to the insulating DC-DCcircuit 72 so as to operate the insulating DC-DC circuit 72 (ACT12). Thecontrol circuit 79 controls the pulse width of the pulse signal which isinput to the first switching element SW1 by the first PWM pulsegenerator 81, and thus holds the output voltage of the insulating DC-DCcircuit 72 to be constant.

The control circuit 79 inputs a control signal to the DC-DC circuit 78so as to suspend an operation of the DC-DC circuit 78 (ACT13). Thus, thecontrol circuit 79 performs a control such that electric power for thestand-by module SB is supplied from the insulating DC-DC circuit 72 andnot from the DC-DC circuit 78.

The control circuit 79 inputs a control signal to the heater controlcircuit 73 so as to operate the heater control circuit 73 (ACT14). Thecontrol circuit 79 controls heat generation of the thermal fixing beltBT. Then, the control circuit 79 causes the print medium to bedischarged.

The control circuit 79 inputs a control signal to the A-CC circuit 75 soas to operate the A-CC circuit 75 (ACT15). The control circuit 79designates the ON duty cycle of an initial A-CC driving pulse to theA-CC circuit 75.

The control circuit 79 acquires the current value of the generated powercurrent detected by the current detection circuit A of the A-CC circuit75, while changing the ON duty cycle of the A-CC driving pulse in theA-CC circuit 75 (ACT16). The control circuit 79 determines whether thiscorresponds to Case 1, Case 2, or Case 3, based on a change of theacquired current value of the generated power current (ACT17). That is,the control circuit 79 determines whether to increase, maintain, ordecrease the ON duty cycle of the A-CC driving pulse.

When the control circuit 79 determines that conditions correspond toCase 1, the control circuit 79 increases the ON duty cycle of the A-CCdriving pulse (ACT18). Specifically, the control circuit 79 calculatesON duty cycle after update Dr (Dr=D+Δd), based on the current ON duty D.

When the control circuit 79 determines that conditions correspond toCase 2, the control circuit 79 maintains the ON duty cycle of the A-CCdriving pulse (ACT19). Specifically, the control circuit 79 sets the ONduty cycle after the update Dr, to be D.

When the control circuit 79 determines that conditions correspond toCase 3, the control circuit 79 decreases the ON duty cycle of the A-CCdriving pulse (ACT20). Specifically, the control circuit 79 calculatesON duty cycle after update Dr (Dr=D−Δd), based on the current ON duty Dcycle.

If the control circuit 79 calculates the ON duty cycle after the updateDr, the control circuit 79 determines whether or not to suspend theoperation of the heater control circuit 73 (ACT21). For example, whenprinting on a print medium is ended, the control circuit 79 makes adetermination to suspend the operation of the heater control circuit 73.

When the control circuit 79 makes a determination to not suspend theoperation of the heater control circuit 73 (YES in ACT21), the controlcircuit 79 transmits the ON duty cycle after the update Dr to the A-CCcircuit 75 (ACT22), and then causes the process to proceed to ACT16. Thecontrol circuit 79 repeats the processes of ACT16 to ACT22 during aperiod until the operation of the heater control circuit 73 issuspended. Thus, the control circuit 79 performs the processingillustrated in FIGS. 10 and 11. As a result, the control circuit 79 cancontrol the current value of the generated power current to follow theideal current curve.

When the control circuit 79 determines to suspend the operation of theheater control circuit 73 (NO in ACT21), the control circuit 79 suspendsthe operation of the A-CC circuit 75 (ACT23), and ends the processing.

FIG. 13 is a diagram illustrating an example of the operation of thecontrol circuit 79 when electric power is supplied to the stand-bymodule SB. It is assumed that, for example, a DC voltage of 5 V issupplied to the stand-by module SB. When the image forming apparatus 1operates, electric power is supplied in the path of the full-waverectifying circuit 71, the insulating DC-DC circuit 72, and the stand-bymodule SB from the AC power supply E. At this time, the generated powercurrent by the thermoelectric conversion element 74 is taken out by theA-CC circuit 75 and is stored in the secondary battery 76. If apredetermined time elapses from when image forming on a print medium iscompleted, the image forming apparatus 1 is put into the sleep mode.

When the image forming apparatus 1 is in the sleep mode, the controlcircuit 79 suspends the operation of the heater control circuit 73(ACT31).

The control circuit 79 determines whether or not the residual amount ofthe electric power in the secondary battery 76 is equal to or greaterthan a preset threshold, based on the detection result supplied from theresidual amount detection circuit 77 (ACT32).

When the control circuit 79 determines that the residual amount of theelectric power in the secondary battery 76 is equal to or greater thanthe preset threshold (YES in ACT32), the control circuit 79 suspends theoperation of the insulating DC-DC circuit 72 (ACT33). The controlcircuit 79 suspends the operation of the A-CC circuit 75 (ACT34). Thecontrol circuit 79 operates the DC-DC circuit 78 so as to supply theelectric power stored in the secondary battery 76 to the stand-by moduleSB (ACT35). Then, the process proceeds to ACT39. In this manner, whenthe residual amount of power remaining in the secondary battery 76 isequal to or greater than the threshold, the stand-by power supplycircuit 20 controls the circuits to supply the electric power from thesecondary battery 76 to the stand-by module SB.

When the control circuit 79 determines that the residual amount of theelectric power in the secondary battery 76 is smaller than the presetthreshold (NO in ACT32), the control circuit 79 operates the insulatingDC-DC circuit 72 to supply the electric power from the AC power supply Eto the stand-by module SB (ACT36). The control circuit 79 suspends theoperation of the A-CC circuit 75 (ACT37). The control circuit 79suspends the operation of the DC-DC circuit 78 (ACT38) and causes theprocess to proceed to ACT39. In this manner, when the residual amount ofpower in the secondary battery 76 is smaller than the threshold, thestand-by power supply circuit 20 controls the circuits to supply theelectric power from the AC power supply E to the stand-by module SB.

The control circuit 79 determines whether or not the sleep modecontinues (ACT39). When the control circuit 79 determines that the sleepmode continues (Yes in ACT36), the control circuit 79 causes the processto proceed to ACT32. Thus, the control circuit 79 switches a powersource for supplying the electric power to the stand-by module SB,between the secondary battery 76 and the AC power supply while theresidual amount of the secondary battery 76 is continuously monitored.When the control circuit 79 determines that the sleep mode has ended (NOin ACT36), the control circuit 79 ends the processing in FIG. 13.

FIG. 14 is a diagram illustrating a control of the units in the powersupply circuit. The horizontal axis indicates time. Vertical axesrespectively indicate a printing timing by the image forming unit 17, acontrol timing of the heater control circuit 73, a control timing of theinsulating DC-DC circuit 72, a control timing of the A-CC circuit 75, acontrol timing of the DC-DC circuit 78, a change of the residual amountof charge or power in the secondary battery 76, and a supply source forsupplying the electric power to the stand-by module SB.

In this example, it is assumed that printing is not performed in aperiod from a time t20 to a time t21 and printing is performed on sixsheets in a period from the time t21 to a time t22. In addition, it isassumed that printing is not performed in a period from the time t22 toa time t24 and printing is performed on three sheets in a period fromthe time t24 to a time t25.

The stand-by power supply circuit 20 operates in the sleep mode in theperiod from the time t20 to the time t21.

If a printing instruction is input, the control circuit 79 starts anoperation of the heater control circuit 73 at the time t21 and startspreparation for printing. If the temperature of the thermal fixing beltBT is equal to or higher than a predetermined temperature, printing isperformed in the period from the time t21 to the time t22. If thetemperature difference in the thermoelectric conversion element 74 isequal to or greater than a predetermined value, the control circuit 79causes the A-CC circuit 75 to store power generated by thethermoelectric conversion element 74 in the period from the time t21 tothe time t22 That is, the control circuit 79 operates the A-CC circuit75 in accordance with the operation of the heater control circuit 73. Ifthis state continues, electric power is charged into the secondarybattery 76, and then electric power charged in the secondary battery 76becomes equal to or greater than a threshold value. If the printing iscompleted and generated power is not sufficiently taken from thethermoelectric conversion element 74, the control circuit 79 suspendsthe operation of the A-CC circuit 75.

The stand-by power supply circuit 20 switches an operation mode of theMFP to the sleep mode from the time t22 when a predetermined timeelapses from when the printing is completed. Thus, the control circuit79 suspends the operation of the heater control circuit 73 and suspendsthe operation of the insulating DC-DC circuit 72. In the example in FIG.14, the control circuit 79 operates the DC-DC circuit 78 to cause theresidual amount of the electric power in the secondary battery 76 to beequal to or greater than a threshold. Thus, the supply source forsupplying electric power to the stand-by module SB is switched from theAC power supply E to the secondary battery 76 at the time t22.

The control circuit 79 sequentially confirms whether or not the residualamount of power in the secondary battery 76 is equal to or greater thanthe threshold. In the example in FIG. 14, the control circuit 79determines that the residual amount of power in the secondary battery 76is smaller than the threshold, at the time t23. In this case, thecontrol circuit 79 starts the operation of the insulating DC-DC circuit72 and suspends the operation of the DC-DC circuit 78. Thus, the supplysource for supplying electric power to the stand-by module SB isswitched from the secondary battery 76 to the AC power supply E at thetime t23.

If a printing instruction is input again, the control circuit 79 startsthe operation of the heater control circuit 73 at the time t24 andstarts preparation for printing. If the temperature of the thermalfixing belt BT is equal to or higher than a predetermined temperature,printing is performed in the period from the time t24 to the time t25.If the temperature difference in the thermoelectric conversion element74 is equal to or greater than a predetermined value, the controlcircuit 79 causes the A-CC circuit 75 to take power generated by thethermoelectric conversion element 74 during the period from the time t24to the time t25. Thus, electric power is charged into the secondarybattery 76, and then the electric power charged into the secondarybattery 76 becomes equal to or greater than the threshold. If theprinting is completed and the generated power is not sufficiently takenout from the thermoelectric conversion element 74, the control circuit79 suspends the operation of the A-CC circuit 75.

The stand-by power supply circuit 20 switches an operation mode of theMFP to the sleep mode from the time t25 when a predetermined timeelapses from when the printing is completed. Thus, the control circuit79 suspends the operation of the heater control circuit 73 and suspendsthe operation of the insulating DC-DC circuit 72. In the example in FIG.14, the control circuit 79 operates the DC-DC circuit 78 to cause theresidual amount of the electric power in the secondary battery 76 to beequal to or greater than a threshold, i.e., the secondary battery ischarged. Thus, the supply source for supplying electric power to thestand-by module SB is switched from the AC power supply E to thesecondary battery 76 again at the time t25.

According to the stand-by power supply circuit 20 configured asdescribed above, the thermoelectric conversion element 74 generateselectric power from the heat of a paper and toner heated by the thermalfixing unit F when printing is performed. The power generated using theheat of the thermoelectric conversion element 74 is taken out by theA-CC circuit 75. Thus, the stand-by power supply circuit 20 can supplyelectric power generated by heat to the stand-by module SB. The controlcircuit 79 of the stand-by power supply circuit 20 controls the A-CCcircuit 75 that takes current out from the thermoelectric conversionelement 74, based on the change of the current taken out from thethermoelectric conversion element 74. That is, the control circuit 79controls the target value of the constant current taken out by the A-CCcircuit 75, based on the change of the value of the current flowing fromthe thermoelectric conversion element 74 into the A-CC circuit 75 whenthe control circuit 79 controls the A-CC circuit 75 so as to change thetarget value of the constant current to be taken out from thethermoelectric conversion element 74. Thus, the stand-by power supplycircuit 20 can take electric power out from the thermoelectricconversion element 74 with high efficiency.

The A-CC circuit 75 includes the second inductor L2 which is connectedin series to the thermoelectric conversion element 74, the fourthswitching element SW4 which is connected in series to the thermoelectricconversion element 74, and the current detection circuit A which isconnected in series to the thermoelectric conversion element 74 anddetects the current value of the current flowing from the thermoelectricconversion element 74 into the A-CC circuit 75. The A-CC circuit 75includes the fifth switching element SW5 and the fifth capacitor C5which are connected in series at a portion between the connection pointof the second inductor L2 and the fourth switching element SW4 and theother terminal of the fourth switching element SW4. Further, the A-CCcircuit 75 includes the second PWM pulse generator 85 and the firstsynchronous rectification pulse generator 86 which are driver circuitsconfigured to input pulse signals for causing the fourth switchingelement SW4 and the fifth switching element SW5 to turn ON and OFF, tothe fourth switching element SW4 and the fifth switching element SW5,respectively. In this configuration, the control circuit 79 maintainsthe current ON state when the current value detected by the currentdetection circuit A is decreased with the decrease of the ON duty cycleof the pulse signal and the current value detected by the currentdetection circuit A is not changed with the increase of the ON dutycycle of the pulse signal. When the current value detected by thecurrent detection circuit A is decreased with the decrease of the ONduty cycle of the pulse signal and the current value detected by thecurrent detection circuit A is increased with the increase of the ONduty cycle of the pulse signal, the control circuit 79 increases the ONduty cycle. When the current value detected by the current detectioncircuit A is not changed with the decrease of the ON duty cycle of thepulse signal and the current value detected by the current detectioncircuit A is not changed with the increase of the ON duty cycle of thepulse signal, the control circuit 79 decreases the ON duty cycle. Thus,the stand-by power supply circuit 20 can take electric power out fromthe thermoelectric conversion element 74 along the ideal current curvedetermined by the characteristics and the temperature of thethermoelectric conversion element 74.

If electric power generated by heat is consumed by the stand-by moduleSB, the stand-by power supply circuit 20 switches the supply source ofthe electric power so as to supply electric power to the stand-by moduleSB from the AC power supply E. Thus, the stand-by power supply circuit20 can use the electric power of the secondary battery 76 when electricpower remains in the secondary battery 76, and can use electric powerfrom the AC power supply E when electric power does not remain in thesecondary battery 76. Thus, in the stand-by power supply circuit 20, itis possible to improve efficiency of electric power consumption.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A power supply circuit comprising: a thermoelectric conversion element configured to generate electric power when it is differentially heated; an adjustable current circuit configured to draw a current from the thermoelectric conversion element and resultantly output a constant current over a period of time; a voltage conversion circuit configured to output a voltage based on the current output by the adjustable current circuit; and a control circuit configured to control the adjustable current circuit to change a target value of the constant current output by the adjustable current circuit.
 2. The power supply circuit according to claim 1, wherein the control circuit controls the target value of the constant current based on a change of a value of a current output from the thermoelectric conversion element after the passage of a period of time.
 3. The power supply circuit according to claim 2, wherein the adjustable current circuit includes: an inductor connected to the thermoelectric conversion element in series; a first switching element connected to the inductor in series; a current detection circuit connected to the inductor in series and configured to detect the value of the current from the thermoelectric conversion element; a second switching element connected in series between a connection point of the inductor and the first switching element and another terminal of the first switching element; a capacitor connected in series with the second switching element between the second switching element and the another terminal of the first switching element; and a driver circuit configured to output pulse signals to the first switching element and the second switching element, the pulse signals causing one of the first switching element and the second switching element to turn ON and the other of the first switching element and the second switching element to turn OFF.
 4. The power supply circuit of claim 3, wherein the pulse signal supplied to the first switching element has a first portion and a second portion, and the duration of time during which the first and second portions are present in a pulse is adjustable by the driver circuit.
 5. The power supply circuit of claim 4, wherein the control circuit is configured to control the driver circuit to change the pulse durations of the first and second portions in the pulse signal supplied to the first switching element based on a value of the current passing through the first switching element.
 6. The power supply of claim 5, wherein the current detection circuit measures the value of the current passing through the first switching element.
 7. The power supply circuit of claim 6, wherein the control circuit is further configured to generate and transmit a control signal to the driver circuit to: vary the duration of the pulse durations of the first and second portions, during a first time period, in the pulse signal supplied to the first switching element; and in response to the detected current passing through the first switching element in the first time period, in a second time period after the first time period, increase the duration of the first portion of the pulse if the detected current passing through the first switching element increases, maintain the duration of the first portion of the pulse if the detected current passing through the first switching element does not increase or decrease, and reduce the duration of the first portion of the pulse if the detected current passing through the first switching element decreases.
 8. The power supply circuit according to claim 1, further comprising: a battery connected to the output of the adjustable current circuit; an AC power supply; a load circuit; a first converter configured to connect the AC power supply to a load circuit; and a second converter configured to connect the battery to the load circuit; wherein the control circuit is further configured to; operate the first converter to supply electric power from the AC power supply to the load circuit and operate the second converter to suspend the supplying of electric power from the battery to the load circuit when an amount of electric power in the secondary battery is smaller than a preset threshold; and operate the first converter to suspend the supplying of the electric power from the AC power supply to the load circuit and operate the second converter to supply the electric power from the battery to the load circuit when the amount of the electric power in the secondary battery is greater than or equal to the preset threshold.
 9. The power supply circuit according to claim 1, further comprising: a thermal fixing device configured to heat a thermal fixing load using electric power from an AC power supply; wherein the thermoelectric conversion element generates the electric power using heat generated by the thermal fixing device; and the control circuit operates the adjustable current circuit in accordance with the operation of the thermal fixing device.
 10. An image forming apparatus comprising: an image forming unit configured to fix a toner image on a print medium using heat supplied thereto by a thermal fixing load; and a power supply circuit configured to supply electric power to the thermal fixing load; wherein the power supply circuit includes; a thermal fixing device configured to heat the thermal fixing load using electric energy supplied from an AC power supply or from a battery; a thermoelectric conversion element configured to generate electric power using the heat of the thermal fixing device; an adjustable current circuit configured to receive current from the thermoelectric conversion element and output a current at a constant value for a period of time; a voltage conversion circuit configured to output a constant voltage for a period of time based on the current output by the variable constant current circuit; and a control circuit configured to operate the adjustable current circuit in accordance with an operation of the thermal fixing device and to control the adjustable current circuit so as to change a target value of the current to be drawn from the thermoelectric conversion element.
 11. The image forming apparatus according to claim 10, wherein the control circuit controls the target value of the current based on a change of the value of the current from the thermoelectric conversion element after the period of time.
 12. The image forming apparatus according to claim 11, wherein the adjustable current circuit includes: an inductor connected to the thermoelectric conversion element in series; a first switching element connected to the inductor in series; a current detection circuit connected to the inductor in series and configured to detect the current from the thermoelectric conversion element; a second switching element connected in series between a connection point of the inductor and the first switching element and another terminal of the first switching element; a capacitor connected in series with the second switching element between second switching element and the another terminal of the first switching element; and a driver circuit configured to output pulse signals to the first switching element and the second switching element, the pulse signals causing one of the first switching element and the second switching element to turn ON and the other of the first switching element and the second switching element to turn OFF.
 13. The image forming apparatus of claim 12, wherein the pulse signal supplied to the first switching element has a first portion and a second portion, and the duration of time during which the first and second portions are present in a pulse is adjustable by the driver circuit.
 14. The image forming apparatus of claim 13, wherein the control circuit is configured to control the driver circuit to change the pulse durations of the first and second portions in the pulse signal supplied to the first switching element based on a value of the current passing through the first switching element.
 15. The image forming apparatus of claim 14, wherein the current detection circuit measures the value of the current passing through the first switching element.
 16. The image forming apparatus of claim 15, wherein the control circuit is further configured to generate and transmit a control signal to the driver circuit to: vary the duration of the pulse durations of the first and second portions, during a first time period, in the pulse signal supplied to the first switching element; and in response to the detected current passing through the first switching element during the first time period, and in a second time period after the first time period, increase the duration of the first portion of the pulse if the detected current passing through the first switching element increases, maintain the duration of the first portion of the pulse if the detected current passing through the first switching element does not increase, and reduce the duration of the first portion of the pulse if the detected current passing through the first switching element decreases.
 17. The image forming apparatus according to claim 10, further comprising: a battery connected to the output of the adjustable current circuit; an AC power supply; a load circuit; a first converter configured to connect the AC power supply to a load circuit; and a second converter configured to connect the secondary battery to the load circuit; wherein the control circuit is further configured to: operate the first converter to supply electric power from the AC power supply to the load circuit and operate the second converter to suspend the supplying of electric power from the battery to the load circuit when an amount of electric power in the battery is smaller than a preset threshold; and operate the first converter to suspend the supplying of the electric power from the AC power supply to the load circuit and operate the second converter to supply the electric power from the battery to the load circuit when an amount of the electric power in the secondary battery is greater than or equal to the preset threshold.
 18. The image forming apparatus according to claim 10, further comprising: a thermal fixing device configured to heat a thermal fixing load using electric power from an AC power supply; wherein the thermoelectric conversion element generates the electric power using heat generated by the thermal fixing device; and the control circuit operates the adjustable current circuit in accordance with the operation of the thermal fixing device.
 19. A circuit for controlling the current drawn from a thermoelectric conversion device and supplied to a load, comprising: a pair of input terminals and a pair of output terminals, the pair of input terminals connected to the output terminals of the thermoelectric conversion device and the pair of input terminals connected to the input terminals of the load; an inductor connected to one of the pair of input terminals; a first switching element connected in series with the inductor and switchable between a closed position wherein current can flow therethrough and an open position wherein current is blocked from flowing therethrough; a second switching element connected in series with the inductor and switchable between a closed position wherein current can flow therethrough and an open position wherein current is blocked from flowing therethrough; a capacitor connected in parallel with the first and second switching elements and across the output terminals; a current detection circuit connected in series with the first switching element and the other of the input terminals; and a pulse generator connected to the first and second switching elements and a control circuit, the pulse generator configured to generate pulses capable of opening and closing the first and second switching elements, and the duration of the pulses is selectively variable by the operation of the pulse generator; wherein the control circuit is configured to change the duration of the pulse generated by the pulse generator received by the first switching element to open the first switching element based on the value of the current flowing through the first switching element in the closed position as detected by the current detection circuit.
 20. The circuit according to claim 19, wherein the pulse generator is configured to generate a pulse capable of opening the first switching element and closing the second switching element after the first switching element has been in a closed state for a first time period, and during the first time period, thermal energy from the thermoelectric conversion device is stored in the inductor as magnetic energy, and after the first switching device is opened and the second switching device is closed, the magnetic energy stored in the inductor is stored in the capacitor as electric charge energy. 